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    <p>I suspect that you're confusing Java beans and EJB (Enterprise Java Beans) - these are different concepts. Actually they're almost the same now, but this was not always so - the history is rather confusing.</p> <p>James gives a good explanation of the history of Java beans - they're much older than annotations (which were introduced in Java 1.5). EJBs are also much older, but they have been radically revised and are now basically Java beans with special annotations running in an EJB container.</p> <p>This is actually a perfect example for how useful annotations can be. </p> <p>"Old style" EJBs (prior to version 3 of the spec) were <em>horrible</em> to code. You needed to define (IIRC) two interfaces, one implementation class (that did not actually implement the interfaces) and an XML descriptor that linked them together. And if you made a typo anywhere, there was no compiler error - just a totally cryptic runtime error that did not help you narrow down the problem.</p> <p>Why was this so? Because it allowed the EJB container to control how the actual implementation code was called, and to transparently do things like access control, transactions and replication.</p> <p>In the EJB 3.0 spec, this was simplified radically, so that now you need only one class (which can be a "classic" Java Bean in the case of entity beans), which actually implements the EJB's logic - and annotations that tell the EJB container how to treat it. Instead of a separate XML file, information about the code lives right next to the code itself in the same file, and since annotation syntax is checked by the compiler, many potential errors are caught at compile time.</p>
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    1. POJava beans, annotations: What do they do? How do they help me?
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    1. USMichael Borgwardt
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    1. COok, so I think what you are saying is there are two kinds of beans. In EJBs, the annotations are processed by the compiler but otherwise they are not? Does this mean that to compile an EJB you need a special compiler? Thanks for the help.
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    2. CONo, EJB annotations are processed by the EJB container at runtime. But they're still part of the code and are checked for syntax errors by the compiler (which then puts them into the class files, where they're accessible through reflection). Old-style EJBs had their metadata in XML descriptors, which the Java compiler obviously did not know about and thus could not check.
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    3. COAnnotations *can* be used to influence the compiler, e.g. @Override - but this is really a less important use case than annotations used at runtime (because few people write compilers).
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